Microelectromechanical systems (MEMS) are small integrated devices or systems that combine electrical and mechanical components. The components can range in size from the sub-micrometer level to the millimeter level, and there can be any number, from one, to few, to potentially thousands or millions, in a particular system. Historically MEMS devices have leveraged and extended the fabrication techniques developed for the silicon integrated circuit industry, namely lithography, doping, deposition, etching, etc. to add mechanical elements such as beams, gears, diaphragms, and springs to silicon circuits either as discrete devices or in combination with integrated silicon electronics. Whilst the majority of development work has focused on silicon (Si) electronics additional benefits may be derived from integrating MEMS devices onto other existing electronics platforms such as silicon germanium (SiGe), gallium arsenide (GaAs) and, indium phosphide (InP) for RF circuits and future potential electronics platforms such as organic based electronics, nanocrystals, etc.
Within the field of radio frequency integrated circuits (RFIC) high quality filters, resonators, oscillators are required and typically must be implemented as bulky off-chip surface-acoustic wave (SAW) filters to achieve a satisfactory quality factor (Q-factor). Whilst current advances in MEMS technology have made it possible to implement such elements on-chip with a comparable Q-factor this is only achieved within the prior art through exploiting proper packaging techniques. Generally, these increase complexity, increase costs, and reduce yields as mechanical/physical sealing of the fabricated MEMS/MEMS-CMOS circuit is required. This is because the Q-factor of a MEMS resonating device is strongly dependent on the level of vacuum in its environment as reducing pressure minimizes air resistance, resulting in reduced damping of the mechanical structure's vibration.
However, the resonating elements of the MEMS are anchored to the substrate which yields anchor damping losses arising from the transfer of kinetic energy from the resonator to its support structure. Mechanical vibrations within a MEMS travel as acoustic or elastic waves which when they impinge upon an interface, such as the resonator—anchor interface then these waves may either be reflected or couple through the interface to the substrate. Those elastic waves that couple to the substrate are lost and hence are an energy loss mechanism for the resonator. Anchor damping can impose considerable losses in MEMS resonators and thus dramatically affect the quality factor. Accordingly, it would be beneficial to reduce anchor damping losses in order to improve the Q factor of resonating MEMS elements.
MEMS resonators are mechanical structures which in order to operate require an electrical input. Their output is a mechanical vibration which is converted into an electrical signal in order to be “sensed” and subsequently utilized. There are several transduction mechanisms that convert mechanical energy into electrical energy and in many instances the choice of the transduction mechanism is an important deciding factor in the MEMS resonator design. Electrostatic and piezoelectric transduction mechanisms are most commonly used due to the ease of fabrication and excellent performance of the designs. However, alternative methods based on optical and magneto-motive transduction do exist, and have met success.
Within the prior art relating to electrostatic transduction significant focus has been made in respect of surface micromachining and reducing the gaps between the actuating electrodes and the sensing electrodes. This focus being due to the fact that the electrostatic coupling coefficient is inversely proportional to the square of the gap. Accordingly, over the past decade many implementations that required DC biases of 150-200V, which is impractical for commercial consumer electronics, have become commercially viable as research on surface micromachining led to gap reduction to dimensions significantly smaller than 1 μm, in some instances down to 30 nm. However, in this design process the electrostatic transduction area has been also limited by the thickness of the material, especially where the resonator is laterally driven, that can be processed at these small dimensions). This being addition to reduced fabrication yields and in many instances use of fabrication processes that were not amendable to mass-production.
Accordingly, it would be beneficial to establish resonators exploiting a combination of bulk and surface micromachining processes so that whilst the gaps are increased to facilitate manufacturing with high yield the increased thickness of the resonating elements results in the transduction area being significantly increased allowing bias voltages to be reduced to voltages compatible with high volume low cost consumer electronics.
However, in essentially all applications, the important considerations for selecting a MEMS sensor include:                Accuracy;        Repeatability;        Long-term stability;        Ease of calibration;        Size;        Packaging; and        Cost effectiveness.        
MEMS sensors require electronic circuits to either provide excitation and/or bias signals, as in the instance of MEMS resonators, or to convert the MEMS sensor output to a signal for use by other electronics. Silicon CMOS electronics has become the predominant technology in analog and digital integrated circuits. This is essentially because of the unparalleled benefits available from CMOS in the areas of circuit size, operating speed, energy efficiency and manufacturing costs which continue to improve from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In respect of MEMS systems, CMOS is particularly suited as digital and analog circuits can be designed in CMOS technologies with very low power consumption. This is due, on the digital side, to the fact that CMOS digital gates dissipate power predominantly during operation and have very low static power consumption. This power consumption arising from the charging and discharging of various load capacitances within the CMOS gates, mostly gate and wire capacitance, but also transistor drain and transistor source capacitances, whenever they are switched. On the analog side, CMOS processes also offers power savings by offering viable operation with sub-1V power supplies and with μA-scale bias currents.
Accordingly, it would be beneficial whilst designing MEMS resonators and absolute pressure sensors it would be beneficial to establish their designs such that they are compatible with combining the CMOS and MEMS technologies into a single integrated circuit. It would be further beneficial for the processes of manufacturing MEMS resonators and absolute pressure sensors to support the integration of other capacitive sensors for other measurands within a single die and for the MEMS elements to be implemented directly atop silicon CMOS electronics (i.e. above integrated circuits, or above-IC) thereby minimizing footprint, cost, and parasitics.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.